VPN Passthrough

TomEastep

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Virtual Private Networking (VPN)

It is often the case that a system behind the firewall needs to be
able to access a remote network through Virtual Private Networking (VPN).
The two most common means for doing this are IPsec and PPTP. The basic
setup is shown in the following diagram:

A system with an RFC 1918 address needs to access a remote network
through a remote gateway. For this example, we will assume that the local
system has IP address 192.168.1.12 and that the remote gateway has IP
address 192.0.2.224.

If PPTP is being used and you need to have two or more local systems
connected to the same remote server at the same time, then you should be
sure that the PPTP helpers modules are loaded (ip_conntrack_pptp and
ip_nat_pptp or nf_conntrack_pptp and nf_nat_pptp). Using the default
modules file, Shorewall (Lite) will attempt to load these modules when
Shorewall (Lite) is started.

If IPsec is being used, you should configure IPsec to use
NAT Traversal -- Under NAT traversal the IPsec
packets (protocol 50 or 51) are encapsulated in UDP packets (normally with
destination port 4500). Additionally, keep-alive
messages are sent frequently so that NATing gateways between
the end-points will retain their connection-tracking entries. This is the
way that I connect to the HP Intranet and it works flawlessly without
anything in Shorewall other than my ACCEPT loc->net policy. NAT
traversal is available as a patch for Windows 2K and is a standard feature
of Windows XP -- simply select "L2TP IPsec VPN" from the "Type of VPN"
pulldown.

Alternatively, if you have an IPsec gateway behind your firewall
then you can try the following: only one system may connect to the remote
gateway and there are firewall configuration requirements as
follows:

Table 1. /etc/shorewall/rules

ACTION

SOURCE

DEST

PROTO

DPORT

SPORT

ORIGDEST

DNAT

net:192.0.2.224

loc:192.168.1.12

50

DNAT

net:192.0.2.224

loc:192.168.1.12

udp

500

The above may or may not work — your mileage may vary. NAT Traversal
is definitely a better solution. To use NAT traversal:

Table 2. /etc/shorewall/rules with NAT Traversal

ACTION

SOURCE

DEST

PROTO

DPORT

SPORT

ORIGDEST

DNAT

net:192.0.2.224

loc:192.168.1.12

udp

4500

DNAT

net:192.0.2.224

loc:192.168.1.12

udp

500

If you want to be able to give access to all of your local systems
to the remote network, you should consider running a VPN client on your
firewall. As starting points, see The /etc/shorewall/tunnels
manpage.